In general, a multilayer printed circuit board includes an internal layer and an external layer on both sides of the internal layer. Through holes can be formed in the inner layer connecting wiring on the front side and on the back side of the internal layer. Some of the through holes, can be RFPs. Since the inside of an individual RFP is resin filled, a conductor or wiring can also be positioned directly on the resin fill in the RFP.
A region within about 0.5 millimeters (mm) from the edge of a multilayer printed circuit board is usually not utilized as a wiring region or does not include RFPs for the following reasons. Since an RFP includes a conductor on its inner circumference wall and the RFP is filled with resin, upon heating of the multilayer printed circuit board, a stress can be generated in a Z-direction (direction perpendicular to a principal surface of a multilayer printed circuit board) because resin and copper differ in their coefficient of thermal expansion. An RFP arranged near the edge of a multilayer printed circuit board, can experience microcracking due to the thermal stress when the multilayer printed circuit board is heated in high-temperature high-humidity test, solder reflow processing or even during operation. Microcracking can also occur in the layers in the vicinity of the RFP. When microcracks are generated, moisture permeation can occur and copper migration is likely.
In the case of a Chip Scale Package (CSP) wherein the size of a multilayer printed circuit board is equal to or slightly greater than the size of a semiconductor chip mounted thereon, inasmuch as wiring area is limited, the necessity for having an RFP and/or wiring as close as possible to the edge of the multilayer printed circuit board has been increasing.
Accordingly there is a need in the art for an improved multilayer printed circuit board wherein a buried-type through hole such as an RFP and/or wiring can be located near the edge thereof, by overcoming the disadvantages of the known structure.
An object of the present invention is to provide a multilayer printed circuit board having a resin filled plated through hole with lands near the edge of the printed circuit board, the plated through hole and multilayer printed circuit board having a much lower tendency for cracking under stress.